The present invention relates to a method and apparatus for detecting foreign matters in a gas sealed electrical apparatus.
One of the problems associated with sealed type gas insulated electrical apparatus is the presence of foreign matters, such as metallic particles, in the electric apparatus. Such foreign matters may be produced by friction or sliding at the contacting surfaces of metallic parts during assembly in the factory or in the site at which the electrical apparatus is installed. The foreign matters may also be introduced while the interior of the electrical apparatus is exposed to the atmosphere before the electrical apparatus is sealed. The presence of the foreign matters in a circuit breaker, a disconnecting switch or a grounding switch is also attributable to local fusion of the movable and stationary contacts due to frictional heat or arcing. If a high voltage is applied to the electrical apparatus with metallic foreign matters contained therein, insulation breakdown occurs at a voltage several times lower than the insulating strength of the apparatus free from any metallic foreign matters or at even lower voltages. With large sized electrical apparatus, it is impossible to accuratily locate the portion of the apparatus where the initial breakdown has occurred. In order to locate the portion of initial breakdown, the whole electrical apparatus must be disassembled, which is time-consuming and laborious.
The insulation breakdown thus caused of course leads to disturbance in the power transmission system. Moreover, the parts damaged by the breakdown need to be disassembled and repaired.
In general, metallic particles contained in sealed type gas insulated electrical apparatus move under electric field and gravity at the bottom of the grounded tank portion in a manner described below. In FIG. 1, an electrical apparatus is shown in a simplified form for brevity of description. The electrical apparatus comprises a cylindrical grounded casing or tank 3 constituting an electrode and a central conductor 1 constituting a second electrode. The space between the casing 3 and the central conductor 1 is filled with an insulating gas 2. Present at the bottom of the tank 3 is a needle-shaped metallic particle 4. When no voltage is applied to the central conductor 1, the metallic particle 4 is at rest, as shown at (a) in FIG. 1. As a voltage is applied to the central conductor 1, electrostatic force is exerted on the metallic particle. As the voltage is increased the electrostatic force is increased. As the electrostatic force overcomes the force of gravity, the metallic particle is made to stand as shown at (b) and (c) in FIG. 1. When the voltage is increased further, the metallic particle moves up, and floats in the gas and reaches the central conductor 1 as shown at (c), (d), (e) in FIG. 1. When the electrostatic force becomes smaller than the force of gravity, or when the direction of the electrostatic force is reversed, the metallic particle 4 falls down as shown at (e), (f) and (g) in FIG. 1 and, upon collision with the bottom of the grounded tank 3, generates elastic waves, essentially consisting of ultrasonic waves. With a needle-shaped metallic particle, the electrostatic force is proportional to the square of the length of the particle, whereas the force of gravity is directly proportional to the length of the particle, so that the longer the particle is, the more active the movement of the particle is, and the greater the generated elastic waves arc. With a globular metallic particle, the electrostatic force is proportional to the square of the radius of the particle, whereas the force of gravity is proportional to the cube of the radius of the particle, so that larger the radius of the particle is, the less active the movement of the particle is. In any case, the metallic part 4 actively moves about in the space between the central conductor 1 and the grounded tank 3, and during the up-and-down movement of the metallic particle insulation breakdown occurs at a voltage much lower than the breakdown voltage in the absense of the metallic particle.
The elastic waves, including the ultrasonic waves, are also caused by corona discharge which occurs at a lower voltage than if no metallic particles are present.
FIG. 2 shows an example of electrical apparatus which may be affected by the presence of metallic particles. A central conductor 1 is supported by a grounded casing 3 through insulating spacers 9 which contain joints 9a for interconnecting adjacent sections of the conductor 1. If globular metallic particles 10 or needle-shaped metallic particles 4 are present, insulation breakdown occurs at a low voltage.
Similar problem also occurs in gas filled circuit breakers.